const [incomeInput, wealthInput, taxInput] = ["incomeInput", 'wealthInput', 'taxInput']
  .map(id => document.getElementById(id));

const tax = (income, wealth) => (0.35 * income) + (0.25 * wealth);

const updateTax = () => {
  const [income, wealth] = [incomeInput.value, wealthInput.value]
    .map(value => parseInt(value))
    .map(value => isNaN(value) ? 0 : value);
  taxInput.value = tax(income, wealth);
}

incomeInput.addEventListener('input', updateTax);
wealthInput.addEventListener('input', updateTax);